WTF does everyone forget Garrosh killed Cairne, Tauren where hating Garrosh way before it was cool to.

This was not a source of hatred for the Tauren. They are not a vengeful people, and frankly, Cairne made a mistake. His death was not murder. If Cairne had won that duel, he would have killed an orc who was innocent of the crimes he accused him of. If Garrosh had known of the poison, then sure, but he didn't. The villain there was Magatha not Garrosh.

This was not a source of hatred for the Tauren. They are not a vengeful people, and frankly, Cairne made a mistake. His death was not murder. If Cairne had won that duel, he would have killed an orc who was innocent of the crimes he accused him of. If Garrosh had known of the poison, then sure, but he didn't. The villain there was Magatha not Garrosh.

Tauren can be vengeful people, it's not a common trait but bloody vengeance can still be in them if their lands and people are attacked, like is shown in the Barrens, tauren see the mess in Camp Taurajo just as a complete butchering of innocent people by Alliance hands, and many of them are very filled with hate for that, there is even an outpost called "Vendetta Point" few steps by Taurajo. About the Garrosh "incident", many tauren indeed understood the whole mess of Magatha behind it, but is very unlikely that many tauren can simply forget that their beloved leader was killed by Garrosh, whatever the reason, even Baine decided to not duel Garrosh and vengeance his father for the good of the Horde. And the death of Cairne has been absolutely a murder, simply Garrosh served as the Magatha's instrument for the murder; Garrosh didn't kill a fighting opponent, he killed an hopeless and defenseless one, even if he could not imagine it.

Indeed the villain was Magatha, but the whole situation showed a trait of Garrosh that is important even for define his character nowdays: he's too easy to manipulate, and is too quick to trust someone that fake to sharing his feelings, even if is someone that shouldn't be trusted at all.

Cairne was killed in an honorable duel though. (At least everyone assumed until it was over.) Not to mention Cairne was the one to challenge Garrosh in the first place. he went into that duel willingly, knowing it would be 'kill or be killed.' Why would the Tauren as a people seek vengeance when they know the real villain behind it was Magatha?

I'd like Blizzard to go really big and bold with the ending of this expansion. Really shake things up, set the foundations for a totally new horde, something we can play and experience ourselves, rather then reading in novels or books.

The razing of Orgrimmar would be an amazing event to experience, and the near destruction of the Horde could breathe new life into a Alliance & Horse pact, that sees them facing the Burning Legion together, as both try to recover from their battles.

A new capital city, perhaps losing Orgrimmar, and turning favour to Thunder Bluff, while Gilneas is lost entirely to the Horde, and Sylvanis becomes a greater threat to all life on Azeroth. I'm no Lore Developer, but what I do know is, I'd like to see something truly, truly epic to end this expansion, like nothing we've seen before!

So you basically want the Alliance to win the war and the Horde can go back to living in huts and cages? Yeah, truly epic when you play Horde!

Cairne was killed in an honorable duel though. (At least everyone assumed until it was over.) Not to mention Cairne was the one to challenge Garrosh in the first place. he went into that duel willingly, knowing it would be 'kill or be killed.' Why would the Tauren as a people seek vengeance when they know the real villain behind it was Magatha?

Cairne was killed because he was betrayed, in a "honorable duel" turned into a farce. Garrosh would have been likely killed by Cairne in that duel without the poison, in the Shattering is wrote that after Garrosh partially destroyed Cairne's spear, Cairne was ready to give the last blows for end the duel, a duel in which he had the upper hand for almost all the time.

Garrosh try hard to be "honorable" as he would love to be, but because of his naivety and pride he involves himself in all the kind of crap situations, and then is forced to endure the shame.

About the Tauren, remember that the question about the Twilight's Hammer is a known fact only for the readers. In game the question has never been solved, so many Tauren probably belive that if Cairne was involved in a Mak'gora in the first place is all Garrosh's fault.

Why would the razing of an entire city, both the Horde and Orc capital, make the Horde feel all nice to the Alliance?

Not the razing itself (As if Blizz would let a faction's capitol get razed anyway), but the inevitable bigger threat looming over the horizon will get the Alliance/Horde war put on the backburner (Where it belongs) for quite some time to come.